In vivo and in vitro aspects of imidazoline-2(1/2)sites within the central nervous system

The in vivo and in vitro characteristics of the I2 binding site were probed using the technique of drug discrimination and receptor autoradiography. Data presented in this thesis indicates the I2 ligand 2-BFI generates a cue in drug discrimination. Further studies indicated agmatine, a proposed endogenous imidazoline ligand, and a number of imidazoline and imidazole analogues of 2-BFI substitute significantly for 2-BFI. In addition to specific I2 ligands the administration of NRl's (noradrenaline reuptake inhibitors), the sympathomimetic d-amphetamine, the α1-adrenoceptor agonist methoxamine, but not the β1 agonist dobutamine or the β2 agonist salbutamol, gave rise to significant levels of substitution for the 2-BFI cue. The administration of the α1-adrenoceptor antagonist WB4101, prior to 2- BFI itself significantly reduced levels of 2-BFI appropriate responding. Administration of the reversible MAO-A inhibitors moclobemide and Ro41-1049, but not the reversible MAO-B inhibitors lazabemide and Ro16-6491, gave rise to potent dose dependent levels of substitution for the 2-BFI cue. Further studies indicated the administration of a number of β-carbolines and the structurally related indole alkaloid ibogaine also gave rise to dose dependent significant levels of substitution. Due to the relationship of indole alkaloids to serotonin the 5-HT releaser fenfluramine and a number of SSRI's (selective serotonin reuptake inhibitor) were also administered and these compounds gave rise to significant partial (20-80% responses to the 2-BFI lever) levels of substitution. The autoradiographical studies reported here indicate [3H]2-BFI labels I2 sites within the rat arcuate nucleus, area postrema, pineal gland, interpeduncular nucleus and subfornical organ. Subsequent experiments confirmed that the drug discrimination dosing schedule significantly increases levels of [3H]2-BFI 12 binding within two of these nuclei. However, levels of [3H]2-BFI specific binding were significantly reduced within four of these nuclei after chronic treatment with the irreversible MAO inhibitors deprenyl and tranylcypromine but not pargyline, which only reduced levels significantly in two. Further autoradiographical studies indicated that the distribution of [3H]2-BFI within the C57/B mouse compares favourably to that within the rat. Comparison of these levels of binding to those from transgenic mice who over-express MAO-B indicates two possibly distinct populations of [3H]2-BFI 12 sites exist in mouse brain. The data presented here indicates the 2-BFI cue is associated with the selective activation of α1-adrenoceptors and possibly 5-HT receptors. 2-BFI trained rats recognise reversible MAO-A but not MAO-B inhibitors. However, data within this thesis indicates the autoradiographical distribution of I2 sites bears a closer resemblance to that of MAO-B not MAO-A and further studies using transgenic mice that over-express MAO-B suggests a non-MAO-B I2 site exists in mouse brain.

Behavioural and neurochemical correlates of drugs acting at imidazoline and a2-adrenoceptor binding sites

A number of agents with differing selectivity profiles for the non-a2 adrenoceptor binding site (NAIBS), imidazoline preferring receptor (IPR) and a2-adrenoceptor were employed in a series of behavioural and neurochemical experiments to determine a functional role for the former two sites. The highly selective NAIBS ligand RX801 077 produced an increase in rat brain extracellular noradrenaline (NA) levels, as determined by the technique of in vivo microdialysis, which may underlie its ability to produce a discriminable cue in the same species. This increase in NA may be due to a suggested link between the NAIBS and the monoamine oxidase inhibitor (MAOI) activity of RX801 077. For instance, the RX801 077 cue was substituted for by the MAOI drugs pargyline and moclobemide, which themselves down regulate NAIBS when administered chronically. RX811 059 substituted for the RX801 077 cue which may be due its ability to stimulate NA release via its activity as a highly selective a2-adrenoceptor antagonist. An effect upon NA output may also explain the ability of RX801 077 to 'mimic' the anti-immobility effect of the antidepressant drug desmethylimipramine (DMJ) in the forced swimming test. Further studies are therefore required to examine a possible role for the NAIBS in the treatment of depression. Discriminable cues were also produced by RX811 059 and the a2- adrenoceptor agonist clonidine, probably as a consequence of their respective ability to stimulate and inhibit NA output via their opposing activity at a2-adrenoceptors. The IPR has been suggested to play a role in mediating the hypotensive effect of clonidine, although a precise role was unable to be established for this site in the present studies due to the unavailability of highly selective IPA agents.